Telecommunications and data communications systems represent a substantial and rapidly growing portion of the communications industry. Such growth has been particularly intense in the past ten years. Unfortunately, the current system capacity is far too limited for these rapidly expanding applications, particularly optical fiber systems. It is quite expensive to construct new fiber optic lines. Thus, various methods have been considered for increase the system capacity, such as optical dense wavelength division multiplexing (DWDM), frequency division multiplexing (FDM), and time division multiplexing (TDM).
In a DWDM system, multiple optical signal channels are carried over a single optical fiber, each channel being assigned a particular optical wavelength. By using a single, existing optical fiber to carry multiple channels, DWDM has been found to be an efficient approach for increasing the capacity of the current communication systems and better utilizing the existing transmission media, optical fiber.
Each channel in current DWDM systems can carry data at speeds up to about 2.5 Gb/s to 10 Gb/s. This works well for data that is being transmitted at such speeds, however it is obviously quite inefficient to use a channel capable of 10 Gb/s to transmit a signal at a substantially lower data rate (often lower than 500 Mb/s). For instance, input signals in the STS-1 format are typically transmitted at about 51.84 Mb/s, while data in the FDDI and Ethernet formats are usually transmitted at about 100 Mb/s, data in the ATM, Fiber Channel and STS-3 formats at about 155 Mb/s, data in the ESCON format about 200 Mb/s, and data in other audio-videodata mixed transmission formats may range from about 150 Mb/s to 200 Mb/s. Thus valuable fiber-optic bandwidth is being employed to transmit far less information than capacity would indicate. What is needed is to find some means to efficiently transport signals with working speeds of about 50 Mb/s to 500 Mb/s on a DWDM system.
TDM modules have been employed in an attempt to avoid such inefficiency. At the transmitting end of a TDM system, an electronic switch (multiplexing unit) picks up signals from each of multiple input channels, in a predetermined "channel-by-channel" order. A resulting multiplexed signal, which combines such input signals, is constructed and forms the "output" or transmitting end of the TDM. Ideally, the output signal is transmitted at the combined speeds of the various input signals. The multiplexed signal is distributed to receiving terminal equipment as the output of transmitting end of the module. Such receiving terminal equipment includes devices made by 3Com, Cisco, Hewlett-Packard, IBM, Alcatel, Amp, Lucent, Tektronix, and Osicom. Generally, output signals are sent to the same kind of terminal equipment as provided the initial input signal.
Prior art TDM modules were designed for to work only with a single, fixed data format. For example, some modules were designed to operate only with data in the FDDI format, while others work only in the Ethernet format, and still others only with one of the remaining formats, such as SDH/SONET ATM, Fiber Channel, and others. Thus systems in which data in a variety of formats must be transmitted cannot use prior art TDM systems. Furthermore, such prior art TDM systems required expensive and complicated format-specific terminal equipment.
Many prior art TDM modules are not capable of adjusting to differing data rates and have to be specially designed for multiplexing at only a single specified rate. Thus such units have limited usefulness in modern communications networks, which often must transmit data of varying data formats and at differing speeds. To overcome this problem, some prior art TDM modules have included a data rate adjustment feature. Unfortunately this is not a true adjustment of the modules data rate, but rather a modification of the input signal to meet some standard data rate. Typically this means that the input signal must either be supplied with redundant data to increase the data rate or some data from the signal will be removed to reduce the signal data rate. The former may work, but at the expense of transmitting unwanted data, wasting scarce waveband, and the later will degrade the performance of the input signal. Another attempt to overcome this problem has been to design units capable of handling either a specified data rate or an even multiple of that rate ("step-by-step"). For example, a unit has been designed that can accept data in both the STS-1 and STS-3 formats (respectively 51.84 Mb/s and 155.52 Mb/s, the later being exactly three times the former). Obviously, such a system is quite limited.
Some prior art systems use a "header" to identify a particular channel. Unfortunately, such headers must typically be lengthily, and thus result in an unnecessarily (and wastefully) high data rate to transmit both the header and the underlying signal before available buffer space has been exhausted.
Furthermore, some prior art units are prone to error when if any of the TDM channels are temporarily inactive. Such problems are often due to the inability to determine the signal horizon in the event of a null signal (repeating zeros) as well as certain other repetitive signals (e.g., repeating 1's and/or 0's: "001100110011 . . . " etc.).
Some of prior art TDM modules require the input signals operate on one-fifth of the data rate which the module components can handle, due to the use of an independent system clock. Such units either operate on lower data rate or require the very expensive working components that are capable of handling such high data rates (i.e., five times the actual data transfer rate).
An additional problem with prior art TDM modules, designed for existing network and terminal equipment, is that any change in equipment, such as replacing the module, required that both the TDM system and the DWDM system be shut down.
What is needed is a TDM expansion module adapted for use in a DWDM system, which can handle data in numerous formats, that is expandable, can handle data at higher transmission speeds (form 50 Mb/s to 500 Mb/s), achieves this result at a minimal cost in terms of data loss, functionality, and reliability.